The tensile strength of the stirrups must be developed for almost their full height.
Clearly, it is impossible to do this by development length.For this reason, stirrups normally are provided with 90o or 135o hooks at their upper end (see back for hook details) and at their lower end, are bent 90o to pass around the longitudinal reinforcement.
In simple spans, or in the positive bending region of continuous spans, where no top bars are required for flexure, stirrups support bars should be used.
According to ACI code for special provision for anchorage of web reinforcement, the ends of single-leg, simple U, or multiple-U stirrups are to be anchored by one of the following means:
For # 5 bars and smaller, and for # 6,7 & 8 bars with fy=40,000 psi or less, a standard hook around longitudinal reinforcement is as shown in figure below.
For # 6,7 & 8 stirrups with fy greater than 40,000 psi, a standard hook around a longitudinal bar, plus an embedment between mid height of the member and the outside end of the hook equal to or greater than in. as shown in figure below.
Each bend shall enclose a longitudinal bar as shown below.
Pairs of U-stirrups or ties so placed as to form a closed unit shall be considered properly spliced when length of laps are 1.3 ld, as shown in figure below.
